****************************************************************************** DORISMail 19-Jul-2005 09:45:20 Message No 0395 ****************************************************************************** Author: "H. Fagard" Subject: Survey of the antenna displacement at Manila Dear Colleagues, In October 2004 we were informed by the host agency of our Manila station that renovation work on the waterproof coating of the terrace where our antenna is installed, had caused one of the feet of the supporting tower to come loose. We have then tried in vain to get more information on a possible antenna displacement resulting from this situation, by asking the host agency to carry out a centering check on the antenna. All we knew from the facts related and pictures provided by the host agency were that the northernmost of the three feet of the antenna's supporting tower might have gone down by a few mm, which would result in a few cm antenna shift towards north. We eventually decided to send a surveyor from IGN in order to accurately survey the new antennna location, and fasten the loose foot while putting back the antenna to its original location as far as possible. This took place at the end of June, and the survey results confirmed our suspicions about an antenna displacement towards north. Here is a summary of the different positions of the 2 GHz phase center, on which the DORIS positionning measurements are actually carried out: Time period East offset North offset 26 Feb 2003 - Oct 2004 0 0 (ref. position) 4 Oct 2004 - 29 Jun 2005 0.018 m 0.066 m (loose foot) 30 Jun 2005 --> 0.007 m -0.003 m (new position) There is no significant difference in the antenna height between the three positions. As some of you might wonder why the new location is not exactly identical to the initial one: it is very easy to accurately center a mark below an antenna, but it is very difficult to center an antenna on an existing mark. When trying to do so, you are lucky if the holes for the expansion bolts end up exactly where you planned them to be, especially on a very uneven concrete + waterproof coating mix. Because the antenna displacement on 4 October 2004 was a sudden one (unlike some gradual shifts due to the corrosion of the base plate, that we experienced at other sites), the above offset values can be used by the analysis centers if they wish to remove the effects of this displacement on their antenna coordinates solutions between 4 Oct 2004 and 29 June 2005. Best regards, Herve Fagard -- ====================================================================== Herve FAGARD e-mail : herve.fagard@ign.fr Institut Geographique National 94165 SAINT-MANDE CEDEX PHONE : + 33 1 43 98 81 48 FRANCE FAX : + 33 1 43 98 84 50 ====================================================================== Please do not reply directly to this message, but send comments and suggestions to IDS.central.bureau@cls.fr